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Tip of the Week: New nport parameter ( dcextrap ) for modeling longer transmission lines

Comments(0)Filed under: RF design, Virtuoso Spectre, Virtuoso Spectre Simulator GXL, Spectre, Spectre RF, Virtuoso Spectre Simulator XL

There is a new nport parameter, dcextrap, available in MMSIM 6.2.1.  The values are constant or unwrap. The default is constant. dcextrap is typically used when the nport s-parameter data file models a system with long delay – and -- the DC point is not included in the s-parameter data file.

One problem that occurs in these systems is the default extrapolation mechanism for the phase calculation results in an incorrect value of phase at DC, and then continues to be incorrect up to Fmin (the lowest frequency data point).

This happens when Fmin does not model the phase well enough at low frequency such that a simple extrapolation provides incorrect results.  Using dcextrap=unwrap can improve the results by calculating a better estimate of the extrapolated phase response at DC. 

For the case of magnitude extrapolation to DC:

With dcextrap=constant (default option), a constant magnitude from the Fmin value
(lowest frequency data point) is used. 

With dcextrap=unwrap, the magnitude is extrapolated from a regression analysis. 
 
For most systems, this works well.  However, we do not recommend the dcextrap parameter be used for high-pass systems, as the magnitude extrapolation may not be ideal. 

Note that dcextrap=unwrap is used only if you *do not* have a DC point in the s-parameter data file.  If there is a DC data point, then this parameter is ignored. Currently this option is available only in spectre standalone and does not appear on the analogLib nport Edit Properties/Add Instance form.  

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